Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” IEEE Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, Dec. 10-12, 1984, pp. 175-179. Specific QKD systems are described in the publication by C. H. Bennett et al., entitled “Experimental Quantum Cryptography,” J. Cryptology 5: 3-28 (1992), in the publication by C. H. Bennett, entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992), and in U.S. Pat. No. 5,307,410 to Bennett (the '410 patent). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33.
The Bennett-Brassard article and the '410 patent each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization of single photons, and Bob randomly measures the polarization of the photons. The one-way system described in the '410 patent is based on a two-part optical fiber Mach-Zehnder interferometer. Respective parts of the interferometer are accessible by Alice and Bob so that each can control the phase of the interferometer. The signals (pulses) sent from Alice to Bob are time-multiplexed and follow different paths. As a consequence, the interferometers need to be actively stabilized during transmission to compensate for thermal drifts. This is generally inconvenient for practical applications involving transmission distances measured in kilometers.
U.S. Pat. No. 6,438,234 to Gisin (the ‘234 patent’), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that employs an autocompensating interferometer of the type invented by Dr. Joachim Meier of Germany and published in 1995 (in German) as “Stabile Interferometrie des nichtlinearen Brechzahl-Koeffizienten von Quarzglasfasern der optischen Nachrichtentechnik,” Joachim Meier.—Als Ms. gedr.—Düsseldorf: VDI-Verl., Nr. 443, 1995 (ISBN 3-18-344308-2). Because the Meier interferometer is autocompensated for polarization and thermal variations, the two-way QKD system based thereon is generally less susceptible to environmental effects than a one-way system.
All QKD systems, regardless of type, require some form of synchronization in order for the system to operate. For example, the activation of various components of the systems, such as the modulators and the detectors, all need to be timed (and gated) relative to the expected arrival times of the quantum signals (photons). The sync signals are also used to establish “qubit buffers” having a length corresponding to a certain number of transmitted qubits (e.g., 104 qubits). Accordingly, the QKD stations Alice and Bob are operatively coupled via a synchronization (“sync”) channel and exchange sync signals over the sync channel.
The sync signal typically has an associated sync signal frame (interval) that, as mentioned above, defines the size of the qubit buffer. In the operation of the QKD system, it is critical that the sync signal interval be kept constant (i.e., error-free) so that the qubit buffers align. In this regard, some QKD systems rely on the use of phase-lock loops (PLLs) to re-clock the sync signal to reduce sync signal errors. Sync signal errors can arise due to a number of reasons, such as too much attenuation of the quantum signal, a bad optical fiber coupling, sideband interference, or a malfunction of a component in either of the two QKD stations.
While conventional communications systems can perform error correction on the sync signals, such correction cannot be performed in a QKD system because a single error in the sync channel signal results in a misalignment of the qubit buffers, which in turn leads to a dramatic increase in the qubit error rate (QBER). In particular, if one of the first bits in the sync channel is missed, and almost all of the sync signal frame is misaligned, a 50% QBER will result. If a single sync signal is missed is near the middle of the frame, the error rate will be at least 25%. This situation makes it impossible to discriminate between an eavesdropper or a missed sync signal, which in effect makes it impossible to exchange keys.